Color mixing for solid state lighting using direct AC drives

Abstract

Techniques are described to allow a low-cost light fixture to produce light with a relatively high color rendering index (CRI) across a range of color temperatures. A rectified AC voltage may be provided to multiple groups of light-emitting diodes (LEDs) having characteristics related to a range of color temperatures and a target CRI level. The multiple groups of LEDs may be controlled based on a received control signal, such that the combined groups of LEDs produce combined light having a produced color temperature within the range of color temperatures, and having a produced CRI level that is approximately the target CRI level. In some implementations, the received control signal is modified by a modification circuit, and the multiple groups of LEDs may be controlled based on the modified control signal.

What is claimed is: 
     
       1. A system for controlling light-emitting diode (LEDs) to produce light having a target color rendering index (CRI) across a range of color temperatures, the system comprising:
 a power source capable of providing a voltage signal comprising a rectified alternating current (AC) signal; 
 multiple LEDs capable of receiving the rectified signal from the power source, wherein:
 a first group of LEDs has a first range of color temperatures and a first CRI value above or equal to a target CRI level, wherein the target CRI provides an accurate perception of color, wherein the first CRI value is constant over the first range of color temperatures, and 
 a second group of LEDs has a second range of color temperatures and a second CRI value different from the target CRI level, wherein a portion of the second range of color temperatures is cooler than the first range of color temperatures, wherein the second CRI value is constant over the second range of color temperatures; and 
 
 a microcontroller capable of receiving the rectified signal and a control signal, wherein the microcontroller is configured to (i) produce a current based on the rectified signal, wherein a level of the current is based on a level of the control signal, (ii) provide the current to one or more of the groups of LEDs, and (iii) control the groups of LEDs to produce light based on the received control signal, such that:
 when the level of the control signal is above a first control threshold and below a second control threshold, the microcontroller provides current having a first level to the first group of LEDs, such that the first group of LEDs produces light having a first color temperature within the first range of color temperatures and having a first CRI level above or equal to the target CRI level, and 
 when the control signal is above the second control threshold and below a third control threshold, the microcontroller provides current having a second level to the first and second groups of LEDs, such that the first and second groups of LEDs produce light having a second color temperature within the second range of color temperatures and having a second CRI level approximately equal to the target CRI level. 
 
 
     
     
       2. The system of  claim 1 , wherein the microcontroller is further configured to provide current to a subset of LEDs included within a particular group of LEDs, such that the particular group of LEDs produces light based on the subset of the LEDs. 
     
     
       3. The system of  claim 1 , wherein when the control signal is above the first control threshold the produced light has a first intensity, and when the control signal is above the second control threshold the produced light has a second intensity higher than the first intensity. 
     
     
       4. The system of  claim 1 , wherein the target CRI level is a CRI level within a range of about 70 to about 100. 
     
     
       5. The system of  claim 1 , wherein the target CRI level is a CRI level of approximately 90. 
     
     
       6. The system of  claim 1 , wherein the first range of color temperatures is between 1800 K and 3300 K and the second range of color temperatures is between 3000 K and 5000 K. 
     
     
       7. The system of  claim 1 , wherein the power source is a rectifier. 
     
     
       8. The system of  claim 1 , wherein the control signal is received from a dimming switch. 
     
     
       9. The system of  claim 1 , wherein the control signal is one of a 0-10 volt dimming signal, a forward phase-cut dimming signal, or a reverse phase-cut dimming signal. 
     
     
       10. The system of  claim 1 , wherein the first CRI value is above the target CRI level, and the second CRI value is below the target CRI level. 
     
     
       11. A method for controlling light-emitting diodes (LEDs) to produce light having a target color rendering index (CRI) across a range of color temperatures, the method comprising:
 receiving, from a power source, a voltage signal comprising a rectified alternating current signal; 
 receiving, from a dimming switch, a control signal having an adjustable level; 
 producing a current based on the rectified signal, wherein a level of the current is based on a level of the control signal; 
 providing the current to multiple groups of LEDs, wherein:
 a first group of LEDs has a first range of color temperatures and a first CRI value above or equal to a target CRI level, wherein the target CRI provides an accurate perception of color, wherein the first CRI value is constant over the first range of color temperatures, and 
 a second group of LEDs has a second range of color temperatures and a second CRI value different from the target CRI level, wherein a portion of the second range of color temperatures is cooler than the first range of color temperatures, wherein the second CRI value is constant over the second range of color temperatures; and 
 
 controlling the groups of LEDs to produce light based on the received control signal, such that:
 when the level of the control signal is above a first control threshold and below a second control threshold, the first group of LEDs produces light having a first color temperature within the first range of color temperatures and having a first CRI level above or equal to the target CRI level, and 
 when the control signal is above the second control threshold and below a third control threshold, the first and second groups of LEDs produce light having a second color temperature within the second range of color temperatures and having a second CRI level approximately equal to the target CRI level. 
 
 
     
     
       12. The method of  claim 11 , wherein when the control signal is above the first control threshold the produced light has a first intensity, and when the control signal is above the second control threshold the produced light has a second intensity higher than the first intensity. 
     
     
       13. The method of  claim 11 , wherein the target CRI level is a CRI level within a range of about 70 to about 100. 
     
     
       14. The method of  claim 11 , wherein the target CRI level is a CRI level of approximately 90. 
     
     
       15. The method of  claim 11 , wherein the first range of color temperatures is between 1800 K and 3300 K and the second range of color temperatures is between 3000 K and 5000 K. 
     
     
       16. The method of  claim 11 , wherein the power source is a rectifier. 
     
     
       17. The method of  claim 11 , wherein the control signal is one of a 0-10 volt dimming signal, a forward phase-cut dimming signal, or a reverse phase-cut dimming signal. 
     
     
       18. The method of  claim 11 , wherein the first CRI value is above the target CRI level, and the second CRI value is below the target CRI level.